Method of and apparatus for testing magnetic objects.



WITNESSES F. P. FAHY;

. METHOD OF AND APPARATUS FOR TESTING MAGNETIC OBJECTS.

APPLICATION FILED SEPT. 23,191.2-

1,223,377.' Patented Apr. 24,1917.

2 SHEETS-SHEET I.

IN VE N TOR A TTORNE Y F.PvFAHY.

METHOD OF AND APPARATUS FOR TESTINGMA GNETIC OBJECTS APPLICATION FILED SEPT. 23, l9l2.

2 SHEETS-SHEET 2.

i i B 5; A 5 hub B I INVENTOR rp v 4 ATTORNEY sociated with the core of the apparatus are provisions for subjecting each branch circuit to the same magneto-motive force and for inductively comparing the magneticefi'ects produced thereby inthe two branch circuits.

This apparatus is characterized by its inherent' simplicity and effectiveness and by the provisions made for subjecting the standard and test pieces to substantially identical magnetic potential differences in successive testing operations of the same character.

The means'employed in p ractice for subjecting the two branches of the magnetic circuit to similar magnetizing forces and ,for obtaining a measure of the flux variations produced thereby in the two magnetic circuits, comprise a magnetizing winding con-- nected to a suitable extraneous source of electro-motive force, andian induced current winding having its terminals connected to a suitable galvanometer or other indicating j include portions instrument. One or the other of these two windings must include two portions,-one applied in inductive relation to one, and the other in inductive relation to the second of the two separate branches of the magnetic core. The second of the two windings may applied partly to one and partly ,to the other of the twobranches of the magnetic core, or may wholly be applied to the common portion of the core.

Instead of employing the apparatus referred to above in a direct comparison of readings obtained when the standard and test pieces are separately employed tobridge nish an indication of the then required one of the air gaps of the apparatus will furnish an indication of the difference between their magnetic characteristics, and when'the apparatus has once .been calibrated, so to speak, by the use of readings obtained with a standard piece, or in some other manner, the readings obtained with a test piece of known shape and size will of themselves furmagnetic characteristics of the pieces teste characteristics possesses the important-practicaladvantage of increasing the sensitiveness of the apparatus, since the quantity to be measured by the galvanometer or. other indlcating instrument may be'the integratedelectromotive force due to the differential of the flux variations in the .two separate magnetic circuits;

standard and test pieces differ slightly in Where the cross section, this variation in cross section may be compensated for by correspondingly varying the number of turns in different portions of the winding applied in part to one and in part to the other of the separate branches of the magnetic circuit.

Practically the same sensitiveness may be imparted to the apparatus, however, when the test piece bridges one air gap and the however, and its specific objects and the advantages possessed by it, reference should be had to theaccompanying drawingsand descriptive matter, 'in'which I have illustrated and described in detail various embodiments.

ofthe invention claimed, and apparatus by which it may be advantageously carried out.

Of the drawings, Figure l is a plan view of one form of apparatus which I may employ with circuit connections thereto diagrammatically illus-' trated.

Fig. 2 is an elevation of ,a portion of the apparatus shown in Fig. 1.

. Fig. 3 is a sectional view of another portion'of the apparatus shown in Fig. 1.

Fig. 4 is a diagrammatic representation of apparatus differing from that shown in Fig. 1 in the arrangement of the windings.

Fig. 5 is a view similar to Fig. 4 illustrating a third, arrangement of the wind,- ings which may be employed.

Figs. 6 to 8 inclusive are views taken similarly to Fig. 1, and each illustrating a different modification ruse of the apparatus shown by Fig; 1, as follows:

Fig. 6 illustrates the use of non-magnetic spacers between the core of the apparatus and thestandard and test pieces.

Fig. 7 illustrates the use of a mutual inv ductance winding to compensate for the re-- moval of the 'standard.piece; and

Fig; .8- illustrates the use of special pole v pieces for testing helical springs. The simultaneous use offstandard and test pieces possessing about the same magnetic .vided with threaded extensions A which pass through the squared central portions B of the legs B, Bwhiphare shown as having cylindrical bodies. The ends of the core slegs B, B are provided with suitable pole piecesB B the; form off which depends upon the shape and character of the articles to be tested and compared. As shown in Figs. 1 and 2, the pole pieces B 'are in the form of disks provided with threaded extensions screwed into the ends of the legs B, B. One of the two articles to be tested or compared, the piece S for instance, which may be regarded as the standard piece, is placed against the two pole pieces B at the left of the cross-bar A, and the article X is placed against the two pole pieces B at the right of the cross-bar. In Figs. 1 and 2 the provisionsshown for securing the pieces 8' and X against the pole pieces comprise vokes I, preferably of non-magnetic material, fitted one over each pole piece, as

clearly shown in Fig. 2. each yoke being provided with a clamping screw, also preferably of non-magnetic material, by means of which the article inclosed by the yoke and pole piece may be pressed against the latter. I may explain at this point that the force with which the article to be testedor compared is pressed against the pole piece does'not seem to be of importance, provided that it is sufficient to insure the contact desired.

In the use of this apparatus each magnetic circuit, including. the standard or test piece, the common core portion A and the corresponding pole piece B and the portions of the core legs B, is subjected to the same magnetizing force and the relative lnductlve effects of the resulting magnetic fluxes in the two branches are determined. In the simple and effective arrangement illustrated in Fig. l for accomplishing this magnetization and determination of the consequent flux variations, a magnetizing coil C is symmetrically disposed upon the cross-bar A and. as shown, is secured against endwise displacement by collars E at the ends of the coil. secured to the cross-bar A by set screws. The terminals of the coil are connected to the terminals of a reversing switch F by means of which the coil may be energized alternately in opposite directions from a source of current indicated conventionally as a battery G. Q represents an amineter, and R, a suitable adjustable resistance in the energizing circuit. D D D and D represent similar test coils placed one upon each of the two branches of the two core legs B at each side of the cross-bar A. These The coils D and D surrounding the pore. tions of the two core legs B in series with" opposes an electro-motive force generated.

at the same time in the coils D anr-lD". \Vhen the magnetic characteristics of the pieces S and X to be compared are alike and the connections between these pieces andthe testing apparatus are similar, as they should be, the integrated electro-motive force generated in the coils D and 1) upon changes in the magnetizing force of the coilC are thus exactly neutralized by the corresponding integrated electro-motive force generated in the coils D and D* and the needle of the ballistic galvanometer or other measuring device remains stationary. Then the magnetic characteristics of the pieces S and X differ so that the flux change in portions of the core portions surrounded by the coils D and D is different from the flux I change in the-core portions surrounded by erty as the hardness for instance of the two artlcles compared, whenthese two articles differ from each other only in a manner affecting this property.

In the usual operation of the apparatus, after the standard and test pieces are put in place the current in the coil C, which has been adjusted togive the desired magnetomotive force, is reversed in direction several times so as to bring the standard and-test pieces into acyclic state magnetically. Then a. readlng of the deflection of the measuring a instrument due to one reversal is taken. This reading, when the constant of the apparatus is known, furnishes an accurate indication of the mechanical properties of the object being tested.

It is desirable for accurate work to the magnetizing current flow through the coil C fairly constant. This involves the keep use of the ammeter Q and adjustments of" the resistance R 1n the partlcular apparatus shown. strength of. the magnetizing current will under certain conditions of flux affect both the standard and test pieces to .a practically equal extent, it is one of the advantages of my invention that a close adjustment of the magnetizing current is unnecessary. Instead of employing a battery G, Fig. 1, as a However, since a variation in the source of magnetizing current, a commercial lighting or power circuit may be employed as'a source, even when the voltage of such a circuit fluctuates considerably.

When particularly accurate results are desired, the results obtained with one setting of the apparatus may be checked by interchanging the standard and test pieces and taking another set of readings. This tends to eliminate errors due to imperfect .pole piece contact; I have already pointed out that there should be substantial identity in shape between the standard and test pieces, but this is to be understood as necessarily applying only to the portions of the pieces between and in proximity to the pole pieces. It will be apparent that the shape and indeed the composition of the portions S and X of the standard and test pieces extending away from the outer sides or ends of the pole pieces B in any setting 'of the apparatus, affect the readings obtained only by virtue of the magnetic leakage from the portions. Leakage sufficient to affect the results obtained willtake place from those portions only of the projecting ends S and X which to the pole pieces are in nnmediate proximity B and if comparatively short portions 01 the projecting ends of the standard piece adjacent the pole pieces are similar to the corresponding portions of the test piece, differences in the outer portions of the projecting ends of the standard and test pieces become unimportant,

By proceeding in the preferred mannerdescribed, the following important and valuable results are obtained: Errors due to basic chemical differences between the standard and test pieces are avoided; errbrs due .to differences of size and shape between the standard and test ieces are avoided. An accurate indication is obtainedof the difference in the magnetic characteristics and mechanical properties dependent thereon or varying therewith in the test piece and a standard piece differing from it only in that it has been annealed to bring it into a certain definite condition.

It is customary in determining the magnetic characteristics of objects by .tests made by an absolute testing method to make them under definite fixed temperature conditions.

' With the above described method of comparison the temperature effect tends to become unimportant, as both the standard and test pieces are affected to practicallyan equal extent by changes in temperature above or below that used ordinarily in making tests by absolute methods.

In place of the coil arrangement shown in Fig. 1, other arrangements of the magnetizing and test coils may be employed. For instance, in .Fig. 4, the magnetizing coil C of Fig. 1 is replaced by four similar magnetizing coils C (1 ,0 C connected in sebranches of the core legs B at each side of the cross-bar A, and the test coils D, D, D, and D of Fig. 1 are replaced by a single test coil D symmetrically disposed upon the cross-bar A and connected to the ballistic galvanometer or other measuring device'H. The magnetizing coils C, C C and C are connected in such manner, that they develop in the magnetic circuit a flux which is closed upon itself as indicated by the arrows. I

- WVhen' the flux change in the pieces S and X is the same, upon a change in the mag netizing force of the coils C, C C C", no flux passes through the common core portion A, since the magnetic potential difference between the ends of the crossl-barA is zero. When thefiux change in the pieces S and X differs, however, a flux will pass through the core portion A, since no longer will there be a zero potential difference between the ends of the core portion A, and the needle indicating device will be deflected. The direction-of needle deflection and its extent will be dependent, of course, upon the difference of the flux changes in the two branches of the magnetic circuit.

Another example of an arrangement of the magnetizing and induced current windings is shown in Fig. 5 wherein the coils (.3', C C and C arranged as in Fig. 4 are surrounded respectively by coils D, D D and D, arranged on the core arms and con-nected up as in Fig. 1.

I prefer to form the magnetic core of the apparatus with a definite gap or gaps symmetrically disposed with respect to the branch circuits. For instance, as shown in Figs. 1 and 3, I divide the core section A at its center and space the divided sections a definite distance apart bya spacer A of, suitable non-magnetic material such as brass. The spacer A is formed with threaded extensions A mechanically connecting the two parts of the core portion A. By providing a high reluctance gap or gaps in the magnetic core of the apparatus, a number of important advantages are obtained. One of these arises from the fact that since the majorvportion of the reluctance-of the magnetic circuit as a whole, when both test and standard pieces are on, is then found in such gap or gaps, the tendency of the gap orgaps is to maintain a practically constant magnetomotive force drop between the ends of the common core portion, and any ordinary'variation in reluctance existing in successive test specimens of the same general character will not modify the magneto-motive force drop except in a very small degree. This makes it possible to subject the objects tested in successive tests to a practically constant magnetic potential difference, without altering had from the fact that the magnetic leakage conditions in the different tests remain practically constant. A third advantage had with thegap or gaps in the magnetic core, is due to the fact that a pole is formed at each gap end at the instant at which the nagnetizingcircuit is broken. These poles act in a manner tending 'to demagnetize both the core of the apparatus proper and the test and standard pieces, thereby lowering the time constant of the magnetic circuit and permitting the-use of an indicator of low time constant. A fourth advantage of the provision of a gap or gaps in the a magnetic core is due to the fact that it mini- 1 mizes the effect of poor contact of either the test or standard piece, or both. by reason of scale or rust on such pieces, since the added reluctance due to such poor contact is but a. small part ofthe total reluctance present. A fifth advantage of the provision of the gap or gaps in the magnetic core is due to the fact that it renders the magnetic quality of the material entering into the construction of the core of minor importance, so that it is possible to construct a number of cores of material which varies slightly in permeability and yet obtain practically uniform results under the same magneto-motive force conditions. 1 I

Preferably I employ a single gap located centrally in the'cross-bar A, as shown in Figs. 1 and 3 since thisis the most advantageous position to control leakage and to provide a hlgher efi'ective magneto-motive force near the pole shoes than at the center of the test or standard bar, so as to compensate for the reluctance due to necessarlly imperfect contact at such shoes and to make the flux in the test and standard pieces more uniform. I may also employ in addition, to, or in lieu of, the gap in the cross-bar A, gaps or non-magnetic spacers, such as brass, Y, between the test and standard pieces and the pole shoes, as shown in Fig. 6. The gaps Y assist in producing uniform flux conditions in the standard and test specimens, and in addition tend to ren-' of a non-magnetic spacer on account of theresultant leakage-I may make the central .core portion A of material of high .relucnetic characteristics of the two pieces may be obtained with the apparatus shown in Fig- 1 by taking readings of the instrument -H with first one and then the second only of the pieces S and T in place.

'VVith one air gap only bridged, however, the quantity measured by the instrument H is much larger and the 'sensitiveness of the apparatus correspondingly less than when both air gaps are bridged as shown in Fig. 1. This may be compensated for, however, as shown in Fig. 7 by the use of a variable y mutual inductance in place of the standard piece. As shown in Fig. 7 the primary of the mutual inductance M is connected in series with the magnetizing coil C. The secondary of the mutual inductance M is connect ed in series with the'coils D and D. In the use of the mutual inductance M the latter is so adjusted that the flux change through the secondarycoil due to a reversal of the current in its. primary coil produces an integrated electro-motive force in the secondary-coilwhich is equal to the integrated the secondary of the mutual inductance are 5 connected in series in such a manner that the sum of them integrated electro-motlve forces opposes the electro-motive force generated at the same time in the coils D and D.

By using a variable mutual inductance in place of a standard piece a number of advantages are gained. First, instead of requiring a standard specimen of the same composition and size as the test specimens, the test specimens may be compared to a fixed standard of flux change. Second, the mutual inductance being variable, it may be calibrated and adjusted so that its settingin any particular test will take into account the area of the test specimen.

In the use of the various forms of apparatus described herein there will be in addition to the flux passing through the two magnetic circuits including the free or polar ends of the core arms B and the common 125 core A which the core is shaped to provide, a leakage flux between the-arms B which does not follow' either of these circuits in its entirety. a 1

The articles which can be tested by use 189 of apparatus of the character described vary greatly, and as I ihave already indicated the character of the apparatus depends to some extent upon the characteristics of the article to be tested and compared.

I Standard and test leaf springs may readily piece.

be compared with the apparatus shown in Fig. 1. The apparatus shown in Fig. 8 is particularly adapted for testing complete helical springs. In the construction shown in this figure each pole piece 13 is formed with a threaded socket coaxial with the corresponding socket of the cooperating pole In each threaded socket is mounted a threaded rod B provided at its lower inner end with a cup-shaped pole piece proper B, receiving the corresponding end of the test or standard spring X or S respectively.

The apparatus disclosed herein while novel with me and useful for many purposes is not claimed herein, but is claimed in my co-pending application, Serial No. 894. filed January 7, 1915. Said co-pending application is to be regarded in part as a division of this application, although the apparatus specifically disclosed therein advantageously differs in some respects from the apparatus disclosed herein. a

While. in accordance' with the provisions of the statute I have disclosed and explained the best forms of my invention nowknown to me, it will be apparent to those skilled in the art that changes may be made in the modes of operation and forms of apparatus disclosed without departing from i the spiritof my invention, and that certain features of the invention may sometimes be used without'a corresponding use of other features.

Having now described my invention, what and measuring the differential of the induction through the two specimens when the magneto-motive force is changed.

2. The method of determining the effect of a heat treatment on a metallic magnetic material which consists in electro-magnetically comparing one specimen of said material which has been subjected to said heat treatment with another specimen of said material of practically the same shape but which is in a thoroughly annealed condition by simultaneously subjecting the two specimens to the effects of the same changing magneto-motive force and measuring the differential of the induction.v through the two specimens when the magnetmmotive force is changed.

3. The method of determining the magnetic properties of a magnetic object which consists in utllizing a changing magnetomotive force to create a changing magnetic 4 fiux,passing one portion of said flux through said object and diverting another portion of said flux away from said object and measuring the differential inductive effects of said flux portions when said magnetomotive force is changed.

FRANK P. FAHY. Witnesses:

ELIZA TONKS, ROBERT S. STUNZ. 

